1. Field of the Invention
The present invention relates to a method and an apparatus of monitoring a polishing process of a semiconductor wafer, which are suitably applied to the well-known Chemical Mechanical Polishing (CMP) process, a method of detecting an endpoint of the polishing process, and a polishing machine equipped with the monitoring apparatus.
2. Description of the Prior Art
To form wiring or interconnecting lines, contact plugs penetrating via holes, and so on, for electronic devices or elements formed on a semiconductor wafer, conventionally, the so-called CMP process has been used. In this case, typically, a dielectric layer is formed on or over the entire wafer to cover the electronic devices or elements and then, a metal layer is formed to overlay the whole dielectric layer. Subsequently, an upper, unnecessary part of the metal layer is globally polished away by a polishing machine until the remaining metal layer has a desired pattern designed for the wiring lines, contact plugs, and so on.
It is important for the CMP process to be monitored for the purpose of detecting an optimum endpoint for the desired pattern at which the polishing operation is stopped. If the degree of polishing is insufficient, in other words, the polishing operation is stopped prematurely, the metal layer tends to be partially left on the underlying dielectric layer, causing electrical short circuit between the wiring lines and/or contact plugs. On the other hand, if the degree of polishing is excessive, in other words, the polishing operation is stopped belatedly, the remaining metal layer tends to have less cross-sections than those desired at the respective wiring lines and contact plugs.
The Japanese Non-Examined Patent Publication No. 7-235520 published in September 1995, which corresponds to the U.S. Pat. No. 5,433,651 issued on July 1995, discloses a technique for monitoring the polishing process of a semiconductor wafer. FIG. 1 shows schematically a prior art polishing process monitoring apparatus using the technique disclosed in the Japanese Non-Examined Patent Publication No. 7-235520.
In FIG. 1, the prior-art in-situ monitoring apparatus is equipped with a circular polishing table 102 rotatable in a horizontal plane, a polishing pad 103 placed on the surface of the table 102, a wafer holder 104 rotatable in a horizontal plane, a laser 106 as a light source for emitting a light beam 105, a photodiode 140 for receiving a reflected light beam 107, and a monitoring means 113. The table 102 has a viewing aperture 138 with a specific size, which allows the incident light beam 105 from the laser 106 to reach a semiconductor wafer or workpiece 101 held onto the bottom surface of the wafer holder 104. A view window 138a is fixed to the aperture 138 to prevent a polishing slurry 116 from flowing out through the aperture 138 while allowing the light beams 105 and 107 to penetrate.
The light beam 105 emitted from the laser 106 is irradiated to the polishing surface of the wafer 101, on which the beam 105 forms a beam spot having a specific diameter. The incident light beam 105 is reflected by the polishing surface of the wafer 101, forming the reflected light beam 107. The reflected light beam 107 is received by the photodiode 140.
The photodiode 140 measures the amount of the reflected light beam 107 and outputs an electric signal s to the monitoring means 113 according to the amount thus measured. The monitoring means 113 samples the electric signal s at specific time intervals to generate an electric detection signal through specific signal processing. Then, the monitoring means 113 displays a time-dependent change of the detection signal on a screen (not shown), in which the ordinate axis is defined as the amount of the detection signal and the abscissa axis as the polishing time.
Next, the operation of the prior-art in-situ monitoring apparatus shown in FIG. 1 is explained below.
The incident light beam 105 emitted from the laser 106 is irradiated through the viewing apertures 138 and 139 and the view window 138a to the polishing surface of the semiconductor wafer 101 held by the wafer holder 104. The irradiated light beam 105 is reflected by the polishing surface of the wafer 101, generating the reflected light beam 107. The reflected light beam 107 travels through the viewing apertures 138 and 139 and the view window 138a to be received by the photodiode 140, in which the amount of the beam 107 is measured and the electric detection signal s is generated according to the amount thus measured. The detection signal s from the photodiode is sampled and averaged in the monitoring means 113, displaying the time-dependent change of the signal s, i.e., the reflected light beam 107. The reflected light beam 107 is generated by xe2x80x9cspecular reflectionxe2x80x9d of the incident light beam 105.
During the time period from the start of polishing to the exposure of the underlying dielectric layer, the strength of the detection signal s, i.e., the amount of the reflected light beam 107, is kept approximately constant. This is because almost all the incident light beam 105 is specularly reflected by the metal layer having a comparatively high reflectance. When the underlying dielectric layer begins to be exposed from the metal layer due to the progressing polishing operation, a part of the incident light beam 105 is specularly reflected by the remaining metal layer and received by the photodiode 140. Thereafter, the amount of the reflected light beam 107 thus received gradually decreases with the progressing polishing operation because of the decreasing surface area of the remaining metal layer. At the same time as this, another part of the incident light beam 105 is specularly reflected by the structure formed below the dielectric layer and received by the photodiode 140. The remainder of the incident light beam 105 is scattered and/or diffracted by the remaining metal layer (i.e., the wiring lines and/or contact plugs) or the structure formed below the dielectric layer, which is not received by the photodiode 140. As a result, after the time the underlying dielectric layer begins to be exposed from the metal layer, the strength of the detection signal s, i.e., the amount of the reflected light beam 107, decreases gradually with time.
At the time when the polishing process reaches a desired endpoint, the dielectric layer is exposed from the remaining metal layer forming the desired wiring lines and/or contact plugs. At this stage, the amount of the reflected light beam 107 has a minimum value. After the time corresponding to the endpoint, the surface-area reduction of the metal layer is substantially zero even if the polishing process further progresses. Thus, the amount of the reflected light beam 107 has substantially a same value as that at the endpoint. In other words, the strength of the detection signal s is kept substantially constant after the corresponding time to the endpoint.
With the prior-art in-situ monitoring apparatus shown in FIG. 1, however, there is a problem that the polishing process may be unable to be monitored correctly according to the material of the semiconductor wafer 101, the thickness of the layered structure on the wafer 101, or the pattern (i.e., geometry or closeness/coarseness) of the wiring lines and/or contact plugs. This problem is due to the following reason.
For example, if the wafer 101 is made of a specific semiconductor material, the reflectance value of the metal layer may have a small difference from that of the underlying layered structure of the wafer 101. In this case, even if the surface area of the metal layer is decreased according to progress of the polishing process, the amount of the reflected light beam 107 (i.e., the strength of the detection signal s) varies only within a narrow range due to the small difference in reflectance. As a result, the endpoint of the polishing process is very difficult or unable to be detected correctly.
Additionally, the Japanese Non-Examined Patent Publication No. 8-174411 published in July 1996 discloses a similar technique to that shown in FIG. 1. In this technique, the amount of a specular-reflected light beam generated by the polishing surface of a semiconductor wafer is monitored during the polishing process. The endpoint of the polishing process is detected based on the change of the amount of a specular-reflected light beam during the process.
Accordingly, an object of the present invention to provide a method and an apparatus of monitoring a polishing process of a semiconductor wafer capable of monitoring correctly the process independent of various factors affecting optical measurement, such as the configuration, material, and size of a layered structure on the wafer, and the geometric shapes of patterns and their arrangement for respective semiconductor chips.
Another object of the present invention to provide an endpoint detection method capable of detecting correctly a desired endpoint of a polishing process of a semiconductor wafer.
Still another object of the present invention to provide a polishing machine capable of monitoring correctly a polishing process independent of various factors affecting optical measurement, such as the configuration, material, and size of a layered structure on the wafer, and the geometric shapes of patterns and their arrangement for respective semiconductor chips.
The above objects together with others not specifically mentioned will become clear to those skilled in the art from the following description.
According to a first aspect of the present invention, a polishing process monitoring apparatus is provided. This apparatus is comprised of (a) a light irradiating means for irradiating a detection light beam to a semiconductor wafer, (b) a first light receiving means for receiving a specular-reflected light beam generated by reflection of the detection light beam at the wafer and for outputting a first signal according to an amount of the specular-reflected light beam, (c) a second light receiving means for receiving a scattered/diffracted light beam generated by scattering or diffraction of the detection light beam at the wafer and for outputting a second signal according to an amount of the scattered/diffracted light beam, and (d) a monitoring means for monitoring a polishing process of the wafer by using the first and second signals.
With the polishing process monitoring apparatus according to the first aspect of the present invention, the first light receiving means outputs the first signal according to the amount of the specular-reflected light beam generated at the wafer and at the same time, the second light receiving means outputs the second signal representing the amount of the scattered/diffracted light beam at the wafer. Therefore, by using at least one of the time-dependent change of the amount of the specular-reflected light beam and that of the scattered/diffracted light beam, the polishing process can be monitored correctly independent of various factors affecting optical measurement, such as the configuration, material, and size of a layered structure on the wafer, and the geometric shapes of patterns and their arrangement for respective semiconductor chips.
According to a second aspect of the present invention, another polishing process monitoring apparatus is provided.
Unlike the apparatus according to the first aspect using the specular-reflected light and scattered/diffracted light beams, the apparatus according to the second aspect uses at least on detection light beam having different wavelengths from one another and at least one specular-reflected light beam. No scattered/diffracted light beam is used.
The polishing process monitoring apparats according to the second aspect is comprised of (a) a light irradiating means for irradiating at least one detection light beam having different wavelengths from one another to a semiconductor wafer, (b) a light receiving means for receiving at least one specular-reflected light beam generated by reflection of the at least one detection light beam at the wafer and for outputting a signal according to an amount of the at least one specular-reflected light beam, and (c) a monitoring means for monitoring a polishing process of the wafer by using the signal.
With the polishing process monitoring apparatus according to the second aspect of the present invention, since the at least one detection light beam having different wavelengths from one another and the at least one specular-reflected light beam are used, the polishing process can be monitored correctly independent on the above-described factors.
According to a third aspect of the present invention, still another polishing process monitoring apparatus is provided, which corresponds to one obtained by adding another light receiving means for receiving a scattered/diffracted light beam generated by scattering or diffraction of the at least one detection light beam at the wafer.
Specifically, the polishing process monitoring apparatus according to the third aspect is comprised of (a) a light irradiating means for irradiating at least one detection light beams having different wavelengths from one another to a semiconductor waver, (b) a first light receiving means for receiving at least one specular-reflected light beam generated by reflection of the at least one detection light beam at the wafer and for outputting a first signal according to an amount of the at least one specular-reflected light beam, (c) a second light receiving means for receiving a scattered/diffracted light beam generated by scattering or diffraction of the at least one detection light beam at the wafer and for outputting a second signal according to an amount of the scattered/diffracted light beam, and (d) a monitoring means for monitoring a polishing process of the wafer by using the first and second signals.
With the polishing process monitoring apparatus according to the third aspect of the present invention, because of the same reason as that shown in the apparatus according first or second aspect, the polishing process can be monitored correctly independent of the above-described factors.
According to a fourth aspect of the present invention, a further polishing process monitoring apparatus is provided.
Unlike the apparatuses according to the first to third aspects, the apparatus according to the fourth aspect includes a light condensing means for condensing a detection light beam.
Specifically, the apparatus according to the fourth aspect is comprised of (a) a light irradiating means for irradiating a detection light beam, (b) a light condensing means for condensing the detection light beam to form a condensed light beam having a spot size smaller than a specific pattern size on the wafer, the light condensing means being located on an optical axis of the detection light beam, (c) a light receiving means for receiving a specular-reflected light beam generated by reflection of the condensed light beam at the wafer and for outputting a signal according to an amount of the specular-reflected light beam, and (d) a monitoring means for monitoring a polishing process of the wafer by using the signal.
With the polishing process monitoring apparatus according to the fourth aspect of the present invention, because of the same reason as that shown in the apparatus according first or second aspect, the polishing process can be monitored correctly independent of the above-described factors.
Additionally, since the detection light beam is condensed prior to irradiation to the wafer, a scattered/diffracted light beam is likely to be generated, increasing the change of the amount of the scattered/diffracted light beam. Thus, there is an additional advantage that process monitoring by using the scattered/diffracted light beam is facilitated.
In the apparatus according to the fourth aspect, the light irradiating means may irradiate a plurality of detection light beams.
According to a fifth aspect of the present invention, a polishing process monitoring method is provided, which corresponds to the apparatus according to the first aspect of the present invention.
The method according to the fifth aspect is comprised of the steps of (a) irradiating a detection light beam to a semiconductor wafer, (b) receiving a specular-reflected light beam generated by reflection of the detection light beam at the wafer to output a first signal according to an amount of the specular-reflected light beam, (c) receiving a scattered/diffracted light beam generated by scattering or diffraction of the detection light beam at the wafer to output a second signal according to an amount of the scattered/diffracted light beam, and (d) processing the first and second signals to produce a resultant signal required for monitoring a polishing process of the wafer.
With the polishing process monitoring method according to the fifth aspect of the present invention, because of the same reason as shown in the polishing process monitoring apparatus according to the first aspect of the present invention, there is the same advantage as that of the apparatus according to the first aspect.
According to a sixth aspect of the present invention, another polishing process monitoring method is provided, which corresponds to the apparatus according to the second aspect of the present invention.
The method according to the sixth aspect is comprised of the steps of (a) irradiating at least one detection light beam having different wavelengths from one another to a semiconductor wafer, (b) receiving at least one specular-reflected light beam generated by reflection of the at least one detection light beam at the wafer and for outputting a signal according to an amount of the at least one specular-reflected light beam, and (c) processing the signal to produce a resultant signal required for monitoring a polishing process of the wafer.
According to a seventh aspect of the present invention, still another polishing process monitoring method is provided, which corresponds to the apparatus according to the third aspect of the present invention.
The method according to the seventh aspect is comprised of the steps of (a) irradiating at least one detection light beam having different wavelengths from one another to a semiconductor wafer, (b) receiving at least one specular-reflected light beam generated by reflection of the at least one detection light beam at the wafer and for outputting a first signal according to an amount of the at least one specular-reflected light beam, (c) receiving a scattered/diffracted light beam generated by scattering or diffraction of the at least one detection light beam at the wafer and for outputting a second signal according to an amount of the scattered/diffracted light beam, and (d) processing the first and second signals to produce a resultant signal required for monitoring a polishing process of the wafer.
According to an eighth aspect of the present invention, a further polishing process monitoring method is provided, which corresponds to the apparatus according to the fourth aspect of the present invention.
The method according to the eighth aspect is comprised of the steps of (a) irradiating a detection light beam, (b) condensing the detection light beam to form a condensed light beam having a spot size smaller than a specific pattern size on the wafer, the light condensing means being located on an optical axis of the detection light beam, (c) receiving a specular-reflected light beam generated by reflection of the condensed light beam at the wafer and for outputting a signal according to an amount of the specular-reflected light beam, and (d) processing the signal to produce a resultant signal required for monitoring a polishing process to the wafer.
In the method according to the eighth aspect, a plurality of detection light beams may be used.
At least two ones of the polishing process monitoring methods according to the fifth to eighth aspects may be combined together as necessary.
In the polishing process monitoring apparatus and methods according to the first to eighth aspects of the present invention, as the detection light beam, any coherent light beam generated by a laser may be preferably used. However, any incoherent light beam generated by a Light-Emitting Diode (LED), halogen lamp, or the like may be used.
The detection light beam may be irradiated to any position of the polishing surface of the wafer if it is always exposed. If the position to be irradiated is located near the center of the wafer, the detection light beam may be screened by the moving polisher. In this case, therefore, the momentary location and the timing of the polisher need to be detected by a position sensor or the like to detect a reflected light beam only when the detection light beam is reflected by the wafer, not by the polisher.
To average the effect of closeness and coarseness of the patterns in each of Integrated Circuit (IC) chips contained in the wafer, the diameter of the detection light beam is preferably set in such a way as to have a spot size equal to or greater than the size of the chips contained in the wafer. However, if the above-described effect of closeness and coarseness of the patterns can be decreased sufficiently by averaging the first signal (or the first and second signals) during a single rotation of the wafer, the spot size of the detection light beam may be less than the chip size. When the spot size of the detection light beam is less than the chip size, the irradiated position of the wafer may be scanned or switched to average the above-described effect of closeness and coarseness of the patterns.
The detection light beam and the light receiving face of each light receiving means may have any shape, such as circle, rectangular, and so on.
A plurality of detection light beams having different wavelengths may be irradiated along the same optical axis to the wafer. In this case, the detection light beams produce the specular-reflected light beams and the scattered/diffracted light beams, which are separated by a spectrum analyzer to be inputted into the monitoring means. Thus, a first set of signals corresponding to the amount of the specular-reflected light beams and a second set of signals corresponding to the amount of the scattered/diffracted beams are generated. Monitoring of a polishing process of the wafer is carried out by using the first and second sets of signals.
As the spectrum analyzer, a wavelength-selecting filter, a wavelength-selecting mirror, or a diffraction grating may be preferably used.
To realize a plurality of detection light beams having different wavelengths, a plurality of lasers oscillating at a single wavelength typically used. However, a multi-line laser capable of oscillating at different wavelengths may be used. In this case, a single light bean containing different wavelengths is produced.
The detection light beam may be condensed by an optical condensing means to a specific pattern size and irradiated to the wafer.
The specular-reflected light beam may be directly received by the first light receiving means. It may be indirectly received by the first light receiving means through a mirror or the like.
The scattered/diffracted light beam may be condensed by an ellipsoidal mirror located on the optical axis of the specular-reflected light beam. The size of the light-receiving face for the scattered/diffracted light beam is preferably wider than that for the specular-reflected light beam. The light receiving face for the scattered/diffracted light beam is preferably located on the optical axis of the specular-reflected light beam at a downstream position with respect to the light receiving or reflecting means for the specular-reflected light beam.
As the light receiving means for the specular-reflected light beam and/or the scattered/diffracted light beam, any light receiving element such as a photodiode, and a photomultiplier may be used.
To remove selectively the polishing slurry from the detection area of the wafer to thereby form a window in the slurry, allowing the specular-reflected light beam to be formed from the detection light beam, preferably, any fluid (i.e., gas or liquid) is emitted to a specific location of the wafer at a specific speed and a specific flowing rate. Although the emitted fluid is typically directed to a position for forming the window of the slurry, it may be directed to another position apart from the position (i.e., detection area) for the window by a specific distance in a specific direction.
To emit the fluid for forming the detection window in the slurry, a nozzle is preferably provided. However, the nozzle may be omitted if the rotation speed of the wafer is high enough for the slurry to be fully spread and to be sufficiently thin on the whole wafer due to the centrifugal force, applying no effect to detection of the specular-reflected light beam.
The position and angle of the nozzle and the fluid pressure emitted from the nozzle are optionally set if they apply no effect to monitoring of the polishing process. If the supply rate of the slurry onto the wafer is greater than the spreading rate of the slurry for forming the window on the wafer due to the high rotation speed of the wafer, the nozzle is preferably located at an upstream position with respect to the window.
An endpoint of the polishing process may be detected by the monitoring means of the apparatus according to one of the first to fourth aspects in any way, some preferred examples of which are explained below.
(i) After a mean or average value of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated, the mean value is compared with a specific threshold value. Then, the time when at least one of the mean values of the two light beams is higher or lower than their threshold values is determined as an endpoint of the polishing process.
(ii) A mean or average value of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated. On the other hand, a mean or average value of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period after a specific time period has been passed from the start of the polishing process is calculated. Then, differences or ratios between the two means values are calculated for the specular-reflected and scattered/diffracted light beams and then, the differences or ratios thus calculated are compared with their specific threshold values. Finally, the time when at least one of the differences or ratios of the two light beams is higher or lower than their threshold values is determined as an endpoint of the polishing process.
(iii) After a mean or average value of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated, the mean value is differentiated by time. The absolute value of the time-differentiated value is compared with a specific threshold value. Then, the time when at least one of the absolute values of the two light beams is lower than their threshold values is determined as an endpoint of the polishing process. Instead of the time-differentiated values, the change of the mean values may be used.
(iv) After a maximum value of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated, the maximum value is compared with a specific threshold value. Then, the time when at least one of the maximum values of the two light beams is higher or lower than their threshold values is determined as an endpoint of the polishing process.
(x) After an amplitude (i.e., the difference between a maximum value and a minimum value) of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated, the amplitude is compared with a specific threshold value. Then, the time when at least one of the amplitudes of the two light beams is higher than their threshold values is determined as an endpoint of the polishing process.
(xi) After a dispersion of the amount of each of the specular-reflected and scattered/diffracted light beams during a specific time period is calculated, the dispersion is compared with a specific threshold value. Then, the time when at least one of the dispersions of the two light beams is higher than their threshold values is determined as an endpoint of the polishing process.
(xii) After a mean or average value of the amount of each of the specular-reflected light beam or beams having different wavelengths and the scattered/diffracted light beam or beams having different wavelengths during a specific time period is calculated, the mean value is compared with a specific threshold value. Then, the time when at least one of the mean values of the two light beams having different wavelengths is higher or lower than their threshold values is determined as an endpoint of the polishing process.
(xiii) A mean or average value of the amount of each of the specular-reflected light beams having different wavelengths during a specific time period is calculated. On the other hand, a mean or average value of the amount of each of the specular-reflected light beams having different wavelengths during another specific time period after a specific time period has been passed from the start of the polishing process is calculated. Then, a difference or ratio between the two mean values is calculated for each of the specular-reflected light beams and then, the difference or ratio thus calculated is compared with a specific threshold value. Finally, the time when at least one of the differences or ratios of the light beams is higher or lower than their threshold values is determined as an endpoint of the polishing process.
(ix) After a maximum value and a mean value of the amount of the specular-reflected light beam during a specific time period are calculated, a difference or ratio between the maximum value and the mean value is calculated. Then, the difference or ratio is compared with a specific threshold value. Finally, the time when the difference or ratio is higher or lower than the threshold value is determined as an endpoint of the polishing process. This is preferred for the case where the detection light beam has a size equal to or less than a specific beam size of the detection light beam is condensed to have a spot size equal to or less than a specific size.
In addition, a difference or ratio of the scattered/diffracted light beam is calculated in the same way as that of the specular-reflected light beam and then, it is compared with a specific threshold value. Subsequently, an endpoint of the polishing process may be determined based on the comparison results for the specular-reflected and scattered/diffracted light beams.
(x) After a mean or average value of the amount of each of the specular-reflected and scattered/diffracted light beams during each of specific time periods is calculated, a variation between maximum and minimum values of the mean values during specific preceding time periods is calculated. Then, the variation of each beam is compared with a specific threshold value. Finally, the time when at least one of the variations of the two beams is higher or lower than the threshold value is determined as an endpoint of the polishing process.
(xi) In the above-described methods (i) to (x), instead of the value or values during each specific time period to be compared with the corresponding threshold value or values, a mean value or values during specific preceding time periods is/are used.
(xii) In the above-described methods (i) to (x), an endpoint is determined as the time when at least one of the values is higher or lower than the threshold value during specific consecutive time periods.
(xiii) In the above-described methods (i) to (x), an endpoint is determined by using the changing state or behavior of each of the values.
(xiv) In the above-described methods (i) to (x), an endpoint is determined as a time delayed by a specific time period from the time when at lest one of the values is higher or lower than the threshold value during a specific time period or specific consecutive timer periods.
(xv) In the above-described methods (i) to (x), instead of the value or values during each specific time period to be compared with the corresponding threshold value or values, a mean value or values during specific preceding or consecutive time periods is/are compared with the corresponding threshold values. Then, an endpoint is determined as a time delayed by a specific time period from the time when at least one of the mean values is higher or lower than the corresponding threshold value.
(xvi) At least two ones of the above-described methods (i) to (xv) are selected and combined together as a logical addition or logical multiplication, thereby determining an endpoint.
(xvii) In the above-described methods (i) to (xvi), an endpoint is determined as the time when the measured or calculated value or values is equal to or greater or less than the corresponding threshold value or values.
According to a ninth aspect of the present invention, a polishing machine is provided, which is comprised of a polishing means for polishing a polishing surface of the semiconductor wafer, and one of the polishing process monitoring apparatuses according to the first to fourth aspects of the present invention.
In the machine according to the ninth aspect, it is preferred that the polishing surface of the wafer faces upward. However, the surface may face any orientation if an optical path (or paths) for detecting the specular-reflected light beam (and for the scattered/diffracted light beam) is (are) formed.